Multimodal polyethylene thin film

ABSTRACT

The present invention relates to a reactor system for a multimodal polyethylene polymerization process, comprising; (a) a first reactor; (b) a hydrogen removal unit arranged between the first reactor and a second reactor comprising at least one vessel connected with a depressurization equipment, preferably selected from vacuum pump, compressor, blower, ejector or a combination, thereof, the depressurization equipment allowing to adjust an operating pressure to a pressure in a range of 100-200 kPa (abs); (c) the second reactor; and. (d) a third reactor and the use of a film thereof.

The present invention relates to a reactor system for a multimodalpolyethylene polymerization process, a process for producing amultimodal polyethylene composition using said reactor system, amultimodal polyethylene composition obtainable this way, and to a filmcomprising said multimodal polyethylene composition.

The demand of polyethylene resins is increasingly being used in avariety of applications. As required high performance of polyethylenefor a relatively new plastic, a polymerization process technology hasbeen developed to support new polymeric material production. In orderfor balancing processability and physical properties of ethylenecopolymers, the development in multimodal polymerization process hasbeen investigated.

In the prior art, multimodal polyethylene polymerization is employed toproduce polymers having different molecular weights by creating eachresin fraction in separated reactors. A low molecular weight fraction isproduced in a reactor using an excess of hydrogen to control themolecular weight of the polymer suitable for providing goodprocessability of the final polymer. A high molecular weight fractionwhich has an influence on the physical properties and is produced underpolymerization conditions with low hydrogen concentration. It is wellknown in the art that low molecular weight polymer is preferablyproduced in a first reactor. To obtain a multimodal polymer with goodphysical properties, all hydrogen from the first reactor should beremoved before the polymerized slurry polymer is passed to a secondreactor in which the production of high molecular weight polymer takesplace.

US2010/0092709 A1 describes a process for preparing bimodal polyethylenecopolymers. The polymerization in a second reactor is operated at a hightemperature with a low comonomer-to-ethylene-ratio and lowhydrogen-to-ethylene-ratio to obtain resins having improved stress crackresistance and melt strength.

U.S. Pat. No. 6,716,936 B1 describes a process for producing bimodalpolyethylene copolymers. A second reactor is operated under hydrogendepleted polyethylene polymerization by directing a polyethylene slurrystream from a first reactor to a hydrogen removal system. Polymerizationin both the first and the second reactors is operated at the bubblepoint by using propane or isobutane as a light solvent. The process issuitable for the production of a bimodal polyethylene for highlyhomogeneous high molecular weight resins.

U.S. Pat. No. 6,291,601 B1 describes a process for producing a bimodalcopolymer with relatively high molecular weight polyethylene. Ahydrogenation catalyst is introduced into a second reactor to consumeresidual hydrogen gas from first reactor by converting hydrogen intoethane leading to a low hydrogen concentration in the second reactor.Using this technique, the cost of raw material consumption of bothhydrogen and ethylene are increased due to converting of unreactedgases.

US 2003/0191251 A1 discloses a process for removing residual hydrogenfrom a polymer slurry by using two flash drums placed between cascadereactors which use light solvent as a diluent. The addition of make-upsolvent to the first flash drum outlet is required to prevent a slurrytransfer pump blocking. Furthermore, warm make-up solvent is necessarybefore transferring slurry into the next flash drum.

EP 1 655 334 A1 discloses the multimodal production of an ethylenepolymer which is produced in a multistage process with a MgCl₂-basedZiegler-Natta catalyst. The polymerization stages are performed in thefollowing order to achieve firstly a ultra high molecular weightpolymer, followed by achieving a low molecular weight polymer, andfinally achieving high molecular weight polymer in the last step. Thepolymerization catalyst is charged to a prepolymerization step to makean ultrahigh molecular weight fraction.

WO 2013/144328 describes a composition of multimodal high densitypolyethylene which is produced using a Ziegler-Natta catalyst for use inmolding applications. A small fraction of ultra-high polyethylene ofless than 15% by weight is produced in a third reactor.

US 2009/0105422 A1 describes a process for producing a multimodalpolyethylene. The polymerization is carried out in three cascadereactors, wherein the molecular weight of the polymer in each reactor iscontrolled by the presence of hydrogen. The concentration of thehydrogen in each reactor is reduced subsequently by providing thehighest hydrogen concentration in the first reactor and the lowesthydrogen concentration in the third reactor.

WO 2013/113797 describes a process for polyethylene preparationcomprising three main subsequent steps of polymerized ethylene and atleast one other α-olefin to get the polyethylene with, respectively, alower molecular weight ethylene polymer, a first higher molecular weightethylene polymer and a second higher molecular weight ethylene polymerin accordance with the sequence of a first reactor, a second reactor anda third reactor.

Even though many processes for preparing multimodal polyethylene areknown and have been described, there is still a need for developing newprocesses for multimodal polymerization, particularly for furtherimproving the mechanical properties of polyethylene compositions.

Therefore, it is the object of the present invention to provide areactor system and a process for preparing multimodal polyethylenesovercoming drawbacks of the prior art, in particular to enhance theperformance of a hydrogen removal unit comprised in such a reactor.

It is an further object to provide a multimodal polyethylene compositionovercoming drawbacks of the prior art, in particular having improvedmechanical properties, such as Charpy index.

A variety of films, which may be applied as the single layer or to thecore or the surface of the multi-layer films, are known in the art.Likewise, a variety of polymer compositions, in particular polyethylenecompositions, for producing such films are described.

WO 2013/144324 A1 discloses a polymer composition comprising ahomopolymer, a first copolymer and a second copolymer of specific MFR₅,density and molecular weight distribution. The polymer composition isprepared in a process involving a slurry loop reactor and two gas phasereactors.

WO 2006/092378 A1 discloses a film prepared from a polymer compositionhaving a specific MFR₅ and density and comprising three constituents,namely a homopolymer and two different copolymers.

US 2015/0051364 A1 is related to a multimodal polyethylene copolymercomprising at least three components and having a specific density andMFR₂₁. At least one of the three components is a copolymer.

US 2010/0016526 A1 is related to a thin film which may be produced frombimodal HDPE polymer having specific density. The composition isprepared by a two stage cascade polymerization with series using a mixedcatalyst system.

However, in light of the above prior art, there is still a need toprovide multimodal polyethylene compositions for preparing films andfilms prepared by using multimodal polyethylene compositions overcomingdrawbacks of the prior art, in particular high density polyethylenecompositions for blown film with improved properties regarding highoutput, good bubble stability, high mechanical strength and hightoughness at film thicknesses from 4 to 40 micron or, preferably, less.

Therefore, it is the further object of the present invention to providemultimodal polyethylene compositions for preparing films and filmsprepared this way overcoming drawbacks of the prior art, in particularovercoming the drawbacks mentioned above.

This object is achieved in accordance with the invention according tothe subject-matter of the independent claims. Preferred embodimentsresult from the sub-claims.

The object is first of all achieved by a reactor system for a multimodalpolyethylene polymerization process, comprising;

-   -   (a) a first reactor;    -   (b) a hydrogen removal unit arranged between the first reactor        and a second reactor comprising at least one vessel connected        with a depressurization equipment, preferably selected from        vacuum pump, compressor, blower, ejector or a combination        thereof, the depressurization equipment allowing to adjust an        operating pressure to a pressure in a range of 100-200 kPa        (abs);    -   (d) the second reactor; and    -   (e) a third reactor.

Preferably, the depressurization equipment allows to adjust theoperating pressure in the hydrogen removal unit to a pressure in therange of 103-145 kPa (abs), preferably 104-130 kPa (abs), mostpreferably 105 to 115 kPa (abs).

Preferably, the hydrogen removal unit further contains a strippingcolumn for the separation of hydrogen and a liquid diluent.

The object is further achieved by a process for producing a multimodalpolyethylene composition in an inventive reactor system, comprising (inthis sequence);

(a) polymerizing ethylene in an inert hydrocarbon medium in the firstreactor in the presence of a catalyst system, selected fromZiegler-Natta catalyst or metallocene, and hydrogen in an amount of0.1-95% by mol with respect to the total gas present in the vapor phasein the first reactor to obtain a low molecular weight polyethylenehaving a weight average molecular weight (Mw) of 20,000 to 90,000 g/molor medium molecular weight polyethylene having a weight averagemolecular weight (Mw) of more than 90,000 to 150,000 g/mol wherein thelow molecular weight polyethylene, respectively the medium molecularweight polyethylene, has a density at least 0.965 g/cm3, and the lowmolecular weight polyethylene has MI2 in the range from 10 to 1,000 g/10min and the medium molecular weight polyethylene has MI2 in the rangefrom 0.1 to 10 g/10 min;

(b) removing in the hydrogen removal unit 98.0 to 99.8% by weight of thehydrogen comprised in a slurry mixture obtained from the first reactorat a pressure in the range of 103-145 kPa (abs) and transferring theobtained residual mixture to the second reactor;

(c) polymerizing ethylene and optionally C4 to C12 α-olefin comonomer inthe second reactor in the presence of a catalyst system, selected fromZiegler-Natta catalyst or metallocene, and in the presence of hydrogenin an amount obtained in step (b) to obtain a first high molecularweight polyethylene having a weight average molecular weight (Mw) ofmore than 150,000 to 1,000,000 g/mol or a first ultra high molecularweight polyethylene in the form of a homopolymer or a copolymer having aweight average molecular weight (Mw) of more than 1,000,000 to 5,000,000g/mol and transferring a resultant mixture to the third reactor; and

(d) polymerizing ethylene, and optionally C4 to C12 α-olefin comonomerin the third reactor in the presence of a catalyst system, selected fromZiegler-Natta catalyst or metallocene, and hydrogen, wherein the amountof hydrogen in the third reactor is in a range of 0.1-70% by mol,preferably 0.1-60% by mol with respect to the total gas present in thevapor phase in the third reactor or optionally substantial absence ofhydrogen to obtain a second high molecular weight polyethylene having aweight average molecular weight (Mw) of more than 150,000 to 1,000,000g/mol or a second ultra high molecular weight polyethylene in the formof a homopolymer or copolymer having a weight average molecular weight(Mw) of more than 1,000,000 to 5,000,000 g/mol.

“Substantial absence” in this regard means that hydrogen is onlycomprised in the third reactor in an amount which cannot be avoided bytechnical means.

The slurry mixture obtained from the first reactor and subjected to thestep of removing hydrogen in the hydrogen removal unit contains all ofthe solid and liquid constituents obtained in the first reactor, inparticular the low molecular weight polyethylene or the medium molecularweight polyethylene. Furthermore, the slurry mixture obtained from thefirst reactor is saturated with hydrogen regardless the amount ofhydrogen used in the first reactor.

Preferably, the removing is removing of 98.0 to 99.8% by weight of thehydrogen, and more preferable 98.0 to 99.5% by weight, most preferred98.0 to 99.1% by weight.

Preferably, the α-olefin comonomer comprised in the second reactorand/or in the third reactor is selected from 1-butene and/or 1-hexene.

Preferably, the operation pressure in the hydrogen removal unit is inthe range of 103-145 kPa (abs) and more preferably 104-130 kPa (abs),most preferred 105 to 115 kPa (abs).

The weight average molecular weight (Mw) of the low molecular weightpolyethylene, the medium molecular weight polyethylene, the highmolecular weight polyethylene and the ultra high molecular weightpolyethylene described herein are in the range of 20,000-90,000 g/mol(low), more than 90,000-150,000 g/mol (medium), more than150,000-1,000,000 g/mol (high) and more than 1,000,000-5,000,000 g/mol(ultra high) respectively.

Finally, the object is achieved by a multimodal polyethylene composition

obtainable by the inventive process, comprising;

(A) 30 to 65 parts by weight, preferably 40 to 65 parts by weight,preferably 43 to 52 parts by weight, most preferred 44 to 50 parts byweight, of the low molecular weight polyethylene, the low molecularweight polyethylene having a weight average molecular weight (Mw) of20,000 to 90,000 g/mol and having a MI₂ from 500 to 1,000 g/10 min,preferably from 600 to 800 g/10 min, according to ASTM D 1238;

(B) 8 to 30 parts by weight, preferably 8 to 20 parts by weight,preferably 10 to 18 parts by weight, most preferred 10 to 15 parts byweight, of the first high molecular weight polyethylene having a weightaverage molecular weight (Mw) of more than 150,000 to 1,000,000 g/mol orthe first ultra high molecular weight polyethylene having a weightaverage molecular weight (Mw) of more than 1,000,000 to 5,000,000 g/mol;and

(C) 30 to 50 parts by weight, preferably 37 to 47 parts by weight, mostpreferred 39 to 45 parts by weight, of the second high molecular weightpolyethylene having a weight average molecular weight (Mw) of more than150,000 to 1,000,000 g/mol or the second ultra high molecular weightpolyethylene having a weight average molecular weight (Mw) of more than1,000,000 to 5,000,000 g/mol, wherein

the density of the first high molecular weight polyethylene or the firstultra high molecular weight polyethylene and the second high molecularweight polyethylene or the second ultra high molecular weightpolyethylene are in the range from 0.920 to 0.950 g/cm3, and

wherein the molecular weight distribution of the multimodal polyethylenecomposition is from 13 to 60, preferably 20 to 60, preferably 20 to 28,preferably from 24 to 28, measured by gel permeation chromatography.

In a further preferred embodiment, the molecular weight distribution isfrom 24 to 26, preferably from 25 to 26, measured by gel permeationchromatography.

In a preferred embodiment, the multimodal polyethylene composition has aweight average molecular weight from 80,000 to 1,300,000 g/mol,preferably 150,000 to 400,000 g/mol, preferably from 200,000 to 350,000g/mol, measured by Gel Permeation Chromatography.

Furthermore, it is preferred, that the multimodal polyethylenecomposition has a number average molecular weight from 5,000 to 30,000g/mol, preferably 5,000 to 15,000 g/mol, preferably from 7,000 to 12,000g/mol, measured by Gel Permeation Chromatography.

Preferably, the multimodal polyethylene composition has a Z averagemolecular weight from 900,000 to 6,000,000 g/mol, preferably 1,000,000to 3,000,000 g/mol, preferably from 1,000,000 to 2,500,000 g/mol,measured by Gel Permeation Chromatography.

Preferably, the multimodal polyethylene composition has a density from0.950 to 0.962 g/cm³ preferably from 0.953 to 0.959 g/cm³, according toASTM D 1505 and/or a melt flow index MI₅ from 0.01 to 50 g/10 min,and/or MI₂ from 0.03 to 0.15 g/10 min preferably from 0.03 to 0.10 g/10min.

Finally, the object is achieved by a film comprising the inventivemultimodal polyethylene composition, wherein the film has a thickness of4 to 40 μm, preferably 4 to 30 μm, and most preferably 4 to 20 μm.

In preferred embodiments of the inventive reactor system, the inventiveprocess, the inventive multimodal polyethylene composition and inventivefilm “comprising” is “consisting of”.

Regarding the inventive film, it is preferred that the filmsubstantially comprises the inventive multimodal polyethylenecomposition, which means that the film does comprise furtherconstituents only in amounts which do not affect the film propertiesregarding output, bubble stability, mechanical strength, toughness andthe like. Most preferred the film is consisting of the inventivemultimodal polyethylene composition.

In preferred embodiments “parts by weight” is “percent by weight”.

The above embodiments mentioned to be preferred resulted in even moreimproved mechanical properties of the obtained multimodal polyethylenecomposition and the film prepared therefrom. Best results were achievedby combining two or more of the above preferred embodiments. Likewise,the embodiments mentioned above to be more or most preferred resulted inthe best improvement of mechanical properties.

Surprisingly, it was found that by using the inventive reactor system toproduce an inventive multimodal polyethylene composition by theinventive process allows to form an inventive film using the inventivecomposition which is superior over the prior art. In particular, it wasfound that by using the inventive multimodal polyethylene composition ablown film can be prepared with high output, good bubble stability, highmechanical strength and high toughness, in particular at a filmthickness from 5 to 12 micron.

The invention concerns a reactor system for multimodal polyethylenepolymerization. The system comprises a first reactor, a second reactor,a third reactor and a hydrogen removal unit placed between the firstreactor and the second reactor.

The hydrogen depleted polyethylene from the first reactor affects thepolymerization of high molecular weight in the subsequent reactors. Inparticular, high molecular weight leads to improved mechanicalproperties of polyethylene that is the advantage for various productapplication includes injection molding, blow molding and extrusion. Thecatalyst for producing the multimodal polyethylene resin of thisinvention is selected from a Ziegler-Natta catalyst, a single sitecatalyst including metallocene-bases catalyst and non-metallocene-basescatalyst or chromium based might be used, preferably conventionalZiegler-Natta catalyst or single site catalyst. The catalyst istypically used together with cocatalysts which are well known in theart.

Innert hydrocarbon is preferably aliphatic hydrocarbon including hexane,isohexane, heptane, isobutane. Preferably, hexane (most preferredn-hexane) is used. Coordination catalyst, ethylene, hydrogen andoptionally α-olefin comonomer are polymerized in the first reactor. Theentire product obtained from the first reactor is then transferred tothe hydrogen removal unit to remove 98.0 to 99.8% by weight of hydrogen,unreacted gas and some volatiles before being fed to the second reactorto continue the polymerization. The polyethylene obtained from thesecond reactor is a bimodal polyethylene which is the combination of theproduct obtained from the first reactor and that of the second reactor.This bimodal polyethylene is then fed to the third reactor to continuethe polymerization. The final multimodal (trimodal) polyethyleneobtained from the third reactor is the mixture of the polymers from thefirst, the second and the third reactor.

The polymerization in the first, the second and the third reactor isconducted under different process conditions. These can be the variationand concentration of ethylene and hydrogen in the vapor phase,temperature or amount of comonomer being fed to each reactor.Appropriate conditions for obtaining a respective homo- or copolymer ofdesired properties, in particularly of desired molecular weight, arewell known in the art. The person skilled in the art is enabled on basisof his general knowledge to choose the respective conditions on thisbasis. As a result, the polyethylene obtained in each reactor has adifferent molecular weight. Appropriate conditions for obtaining arespective homo- or copolymer of desired properties, in particularly ofdesired molecular weight, are well known in the art. The person skilledin the art is enabled on basis of his general knowledge to choose therespective conditions on this basis. Preferably, low molecular weightpolyethylene is produced in the first reactor, while ultra high or andmolecular weight polyethylene are produced in the second and thirdreactor, respectively.

The term first reactor refers to the stage where the low molecularweight polyethylene (LMW) or the medium molecular weight polyethylene(MMW) is produced. The term second reactor refers to the stage where thefirst high or ultra high molecular weight polyethylene (HMW1) isproduced. The term third reactor refers to the stage where the secondhigh or ultra high molecular weight polyethylene (HMW2) is produced.

The term LMW refers to the low molecular weight polyethylene polymerpolymerized in the first reactor having a the weight average molecularweight (Mw) of 20,000-90,000 g/mol.

The term MMW refers to the medium molecular weight polyethylene polymerpolymerized in the first reactor having a number average molecularweight (Mn) of 9,000 to 12,000 g/mol and a weight average molecularweight (Mw) of more than 90,000 to 150,000 g/mol.

The term HMW1 refers to the high or ultra high molecular weightpolyethylene polymer polymerized in the second reactor having a weightaverage molecular weight (Mw) of more than 150,000 to 5,000,000 g/mol.

The term HMW2 refers to the high or ultra high molecular weightpolyethylene polymer polymerized in the third reactor having a weightaverage molecular weight (Mw) of more than 150,000 to 5,000,000 g/mol.

The LMW or MMW is produced in the first reactorin the absence ofcomonomer in order to obtain a homopolymer.

To obtain the improved polyethylene properties of this invention,ethylene is polymerized in the first reactor in the absence of comonomerin order to obtain high density LMW or MMW polyethylene having density≥0.965 g/cm³ and MI₂ in the range of 10-1000 g/10 min for LMW and 0.1 to10 g/10 min for MMW. In order to obtain the target density and MI in thefirst reactor, the polymerization conditions are controlled andadjusted. The temperature in the first reactor ranges from 65-90° C.,preferably 68-85° C. Hydrogen is fed to the first reactor so as tocontrol the molecular weight of the polyethylene. The molar ratio ofhydrogen to ethylene in the vapor phase can be varied depending up onthe target MI. However, the preferred molar ratio ranges from 0.5-8.0,more preferably 3.0-6.0. The first reactor is operated at pressurebetween 250 and 900 kPa, preferably 400-850 kPa. An amount of hydrogenpresent in the vapor phase of the first reactor is in the range of20-95% by mole, preferably 50-90% by mol.

Before being fed to the second reactor, the slurry obtained from thefirst reactor containing LMW or MMW polyethylene preferably in hexane istransferred to a hydrogen removal unit which may have a flash drumconnected with depressurization equipment preferably including one orthe combination of vacuum pump, compressor, blower and ejector where thepressure in the flash drum is reduced so that volatile, unreacted gas,and hydrogen are removed from the slurry stream. The operating pressureof the hydrogen removal unit typically ranges from 103-145 kPa (abs),preferably 104-130 kPa (abs) in which 98.0 to 99.8% by weight ofhydrogen can be removed, preferably 98.0 to 99.5% by weight.

In this invention, when 98.0 to 99.8% by weight of hydrogen is removedand the polymerization undergoes under these conditions of hydrogencontent, very high molecular weight polymer can be achieved this way andCharpy Impact and Flexural Modulus are improved. It was surprisinglyfound that working outside the range of 98.0 to 99.8% by weight ofhydrogen removal, the inventive effect of obtaining very high molecularweight polymer and improving Charpy Impact an Flexural Modulus could notbe observed to the same extend. The effect was more pronounced in theranges mentioned to be preferred.

The polymerization conditions of the second reactor are notablydifferent from that of the first reactor. The temperature in the secondreactor ranges from 70-90° C., preferably 70-80° C. The molar ratio ofhydrogen to ethylene is not controlled in this reactor since hydrogen isnot fed into the second reactor. Hydrogen in the second reactor is thehydrogen left over from the first reactor that remains in slurry streamafter being flashed at the hydrogen removal unit. Polymerizationpressure in the second reactor ranges from 100-3000 kPa, preferably150-900 kPa, more preferably 150-400 kPa and is controlled by theaddition of inert gas such as nitrogen.

Hydrogen removal is the comparison result of the amount of the hydrogenpresent in the slurry mixture before and after passing through thehydrogen removal unit. The calculation of hydrogen removal is performedaccording to the measurement of gas composition in the first and thesecond reactor by gas chromatography.

After the substantial amount of hydrogen is removed to achieve theinventive concentration, slurry from the hydrogen removal unit istransferred to the second reactor to continue the polymerization. Inthis reactor, ethylene can be polymerized with or without α-olefincomonomer to form HMW1 polyethylene in the presence of the LMW or MMWpolyethylene obtained from the first reactor. The α-olefin comomer thatis useful for the copolymerization includes C₄₋₁₂, preferably 1-buteneand/or 1-hexene, more preferably 1-butene.

After the polymerization in the second reactor, the slurry obtained istransferred to the third reactor to continue the polymerization.

The HMW2 is produced in the third reactor by copolymerizing ethylenewith optionally α-olefin comonomer at the presence of LMW and HWM1obtained from the first and second reactor. The α-olefin comonomer thatis useful for the copolymerization include C_(4_12), preferably 1-buteneand/or 1-hexene, more preferably 1-butene.

In order to obtain the target density and the target MI in the thirdreactor, the polymerization conditions are controlled and adjusted.However, the polymerization conditions of the third reactor are notablydifferent from the first and second reactor. The temperature in thethird reactor ranges from 68-90° C. preferably 68-80° C. Hydrogen is fedto the third reactor so as to control the molecular weight ofpolyethylene. The molar ratio of hydrogen to ethylene can be varieddepending up on the target MI. However, the preferred molar ratio rangesfrom 0.01-2.0. Polymerization pressure in the third reactor ranges from150-900 kPa, preferably 150-400 kPa, and is controlled by the additionof inert gas such as nitrogen.

The amount of LMW present in the multimodal polyethylene composition ofthe present invention is 40-65 parts by weight. HMW1 present in thepolyethylene of the present invention is 8-20 parts by weight and HMW2present in the polyethylene of the present invention is 30-50 parts byweight.

The final (free-flow) multimodal polyethylene composition is obtained byseparating hexane from the slurry discharged from the third reactor.

The resultant polyethylene powder may then be mixed with antioxidantsand optionally additives before being extruded and granulated intopellets.

The pellets was then blown into a film using the conventional tubularblow film process with different thickness and further evaluated for thefilm properties.

Definition and Measurement Methods

MI₂, MI₅, MI_(21.6): Melt flow index (MFR) of polyethylene was measuredaccording to ASTM D 1238 and indicated in g/10 min that determines theflowability of polymer under testing condition at 190° C. with load 2.16kg, 5 kg and 21.6 kg, respectively.

Density: Density of polyethylene was measured by observing the level towhich a pellet sinks in a liquid column gradient tube, in comparisonwith standards of known density. This method is determination of thesolid plastic after annealing at 120° C. follow ASTM D 1505.

Molecular weight and Polydispersity index (PDI): The weight averagemolecular weight (Mw), the number average molecular weight (Mn) and theZ average molecular weight (M_(Z)) in g/mol were analysed by gelpermeation chromatography (GPC). Polydispersity index was calculated byMw/Mn.

Around 8 mg of sample was dissolved in 8 ml of 1,2,4-trichlorobenzene at160° C. for 90 min. Then the sample solution, 200 μl, was injected intothe high temperature GPC with IRS, an infared detector (Polymer Char,Spain) with flow rate of 0.5 ml/min at 145° C. in column zone and 160°C. in detector zone. The data was processed by GPC One® software,Polymer Char, Spain.

Intrinsic Viscosity (IV)

The test method covers the determination of the dilute solutionviscosity of HDPE at 135° C. or Ultra High Molecular Weight Polyethylene(UHMWPE) at 150° C. The polymeric solution was prepared by dissolvingpolymer in Decalin with 0.2% wt/vol stabilizer (Irganox 1010 orequivalent). The details are given for the determination of IV followedASTM D2515.

Crystallinity: The crystallinity is frequently used for characterizationby Differential Scanning calorimetry (DSC) follow ASTM D 3418. Sampleswere identified by peak temperature and enthalpy, as well as the %crystallinity was calculated from the peak area.

Charpy impact strength: Charpy impact strength is determined accordingto ISO179 at 23° C., 0° C. and −20° C. and showed in the unit kJ/m².

Flexural Modulus: The specimen was prepared and performed the testaccording to ISO178. The flexural tests were done using a universaltesting machine equipped with three point bending fixture.

Film bubble stability: It was determined during the blown film process,the axial oscillation of the film bubble was observed during increasingthe nip roll take up speed and continue more than 30 minute. Good bubblestability is defined when film is not oscillating and bubble is notbreak.

Output: The film was blown following the blown film conditions. Then thefilm was collected for a minute and weight. The output of film from unitof g/min is then calculated and reported in the unit of kg/hr.

Dart drop impact: This test method follow method A of ASTM D1709 thatcovers the determination of the energy that cause plastic film to failunder specified conditions of free-falling dart impact. This energy isexpressed in terms of the weight of the falling from a specified height,0.66±0.01 m, which result in 50% failure of specimens tested.

Puncture: This testing is in-housed method that a specimen is clampedwithout tension between circular plates of a ring clamp attachment inUTM. A force is exerted against the center of the unsupported portion ofthe test specimen by a solid steel rod attached to the load indicatoruntil rupture of specimen occurs. The maximum force recorded is thevalue of puncture resistance

Tensile and elongation properties of film: The test methods cover thedetermination of tensile properties of film (less than 1.0 mm. inthickness) followed ASTM D882. The testing employs a constant rate ofgrip separation, 500 mm/min.

Tear strength: This test method covers the determination of the averageforce to propagate tearing through a specified length of plastic filmusing an Elmendorf-type tearing tester followed ASTM D 1922

Melt strength and Draw down ratio (DD): They are determined usingGOEFFERT Rheotens. The melt extrudate is performed by single screwextruder with 2 mm die diameter at melt temperature 190° C. theextrudate pass through Rheotens haul-off with controlled the ramp speed.The haul-off force is record. The force(N) is collect as a function ofdraw ratio (DD). Melt strength and draw down ratio is define as theforce at break and draw down ratio at break respectively.

EXPERIMENTAL AND EXAMPLES Composition Related Examples

The medium or high density polyethylene preparation was carried out inthree reactors in series. Ethylene, hydrogen, hexane, catalyst and TEA(triethyl aluminum) co-catalyst were fed into a first reactor in theamounts shown in Table 1. A high activity Ziegler-Natta catalyst wasused. The catalyst preparation is for example described in Hungarianpatent application 0800771R. The polymerization in first reactor wascarried out to make a low molecular weight polyethylene. All ofpolymerized slurry polymer from first reactor was then transferred to ahydrogen removal unit to remove unreacted gas and some of hexane frompolymer. The operating pressure in the hydrogen removal unit was bevaried in a range of 100 to 115 kPa where residual hydrogen was removedmore than 98% by weight but not more than 99.8% by weight from hexanebefore transferring to a second polymerization reactor. Some freshhexane, ethylene and/or comonomer were fed into second reactor toproduce first high molecular weight polyethylene (HMW1). All ofpolymerized polymer from second reactor was fed into the third reactorwhich produce second high molecular weight polyethylene (HMW2).Ethylene, comonomer, hexane and/or hydrogen were fed into the thirdreactor.

Comparative Example 1 (CE1)

A homopolymer was produced in first reactor to obtain a low molecularweight portion before transferring such polymer to hydrogen removalunit. Reactant mixture was introduced into the hydrogen removal unit toseparate the unreacted mixture from the polymer. Residual hydrogen wasremoved 97.6% by weight when hydrogen removal unit was operated atpressure of 150 kPa. The low molecular weight polymer was thentransferred to the second reactor to produce a first high molecularweight polymer. Final, produced polymer from second reactor wastransferred to the third reactor to create a second high molecularweight polymer. In third, a copolymerization was carried out by feeding1-butene as a comonomer.

Example 1 (E1)

Example 1 was carried out in the same manner as Comparative Example 1except that the hydrogen removal unit was operated at pressure of 115kPa. The residual of hydrogen from first reactor was removed 98.0% byweight. Characteristic properties of these multimodal polymers are shownin Table 2. As it can be seen, an improvement of stiffness-impactbalance was observed when the percentage of removed hydrogen residualincreased compared with the properties of Comparative Example 1.

Example 2 (E2)

Example 2 was carried out in the same manner as Comparative Example 1except that the hydrogen removal unit was operated at pressure of 105kPa. The residual hydrogen from the first reactor was removed to anextend of 99.1% by weight. The operational of hydrogen removal unitunder this pressure leads to an expansion of a polymer properties range.As seen in Table 2, a final melt flow rate of E2 was lower than a finalmelt flow rate of CE1 resulted in an improvement of Charpy impact whilestill maintained the flexural modulus.

Comparative Example 2 (CE2)

Comparative Example 2 was carried out in the same manner as ComparativeExample 1 except that the hydrogen removal unit was operated at pressureof 102 kPa. The residual of hydrogen from first reactor was removed toan extend of 99.9% by weight. The operational of hydrogen removal unitunder this pressure leads to an expansion of a polymer properties range.As seen in Table 2, the final melt flow rate and a density of CE2 werequite similar to a final melt flow rate and a density of E2. A decay ofCharpy impact was showed in CE2 compared to E2.

Comparative Example 3 (CE3)

A homopolymer was produced in a first reactor to obtain a low molecularweight portion before transferring the polymer to a hydrogen removalunit. Reactant mixture was introduced into the hydrogen removal unit toseparate the unreacted mixture from the polymer. Hydrogen residual wasremoved to an extend of 97.9% by weight when hydrogen removal unit wasoperated at pressure of 150 kPa. The low molecular weight polymer wasthen transferred to a second reactor to produce a first high molecularweight polymer. In the second reactor, a copolymerization was carriedout by feeding 1-butene as a comonomer. Finally, in-situ bimodalcopolymer from second reactor was transferred to a third reactor tocreate a second high molecular weight copolymer portion. Characteristicproperties of this multimodal polymers is shown in Table 2. Asignificant improvement in Charpy impact at room temperature could beobtained by decreasing a density of final polymer when co-polymer wasproduced in both the second and the third reactor.

Example 3 (E3)

Example 3 was carried out in the same manner as Comparative Example 3except that the hydrogen removal unit was operated at pressure of 105kPa. The residual of hydrogen from first reactor was removed to anextend of 98.8% by weight. The polymer obtained by this processoperation had a melt flow rate of 0.195 g/10 min (5 kg loading) lowerthan such value obtained from CE3. As seen in Table 2, it revealed animprovement of stiffness-impact balance when the percentage of removedhydrogen residual increases compared with the properties of ComparativeExample 3.

Comparative Example 4 (CE4)

A homopolymer was produced in first reactor to obtain a low molecularweight portion before transferring such polymer to hydrogen removalunit. Reactant mixture was introduced into the hydrogen removal unit toseparate the unreacted mixture from the polymer. Residual hydrogen wasremoved 97.6% by weight when hydrogen removal unit was operated atpressure of 150 kPa (abs). The low molecular weight polymer was thentransferred to the second reactor to produce a first high molecularweight polymer. Final, produced polymer from second reactor wastransferred to the third reactor to create a second high molecularweight polymer. In third, a copolymerization was carried out by feeding1-butene as a comonomer. As seen in Table 2 and 3, the final melt flowrate of CE4 were quite similar to a final melt flow rate of E5. A decayof charpy impact and flexural modulus were showed in CE4 compared to E5,even it showed lower density of E5.

Example 5 (E5)

Example 5 was carried out in the same manner as Comparative Example 4except that the hydrogen removal unit was operated at pressure of 115kPa (abs). The residual of hydrogen from first reactor was removed to anextend of 98.5% by weight. The polymer obtained by this processoperation had a melt flow rate of 48 g/10 min (5 kg loading) lower thansuch value obtained from CE3. As seen in Table 2, it revealed animprovement of stiffness-impact balance when the percentage of removedhydrogen residual increases compared with the properties of ComparativeExample 4.

Example 6 (E6)

Example 6 was carried out in the same manner as Example 4 except thatthe comonomer feeding in the third ultra high molecular weightpolyethylene. The polymer produced by this process leads to an excellentimprovement of Charpy impact strength while still maintained theflexural modulus. As shown in table 2, the inventive example 6 with IVof 23 dl/g show the high impact strength (one notched impact withoutbreak) and flexural modulus as compared to comparative samples, however,the melt flow index is unmeasurable due to high viscosity and high Mw.

TABLE 1 CE1 E1 E2 CE2 CE3 E3 E4 CE4 E5 E6 W_(A), % 55 55 55 55 45 45 3050 50 30 W_(B), % 20 20 20 20 25 25 30 10 10 30 W_(C), % 25 25 25 25 3030 40 40 40 40 First reactor Polymerization type Homo Homo Homo HomoHomo Homo Homo Homo Homo Homo Temperature, ° C. 80 80 80 80 80 80 80 8080 80 Total pressure, kPa 800 800 800 800 800 800 800 800 800 800Ethylene, g 1,100.72 1,100.70 1,100.86 1,100.74 900.30 900.30 540.50725.21 725.57 485.70 Hydrogen, g 1.62 1.62 1.55 1.55 2.97 2.99 1.34 1.131.13 1.23 Hydrogen removal unit Pressure, kPa (abs) 150 115 105 102 150105 105 150 115 105 Hydrogen remove, % 97.6 98.0 99.1 99.9 97.9 98.898.9 97.7 98.5 98.3 Second reactor Polymerization type Homo Homo HomoHomo Copo Copo Homo Copo Copo Homo Temperature, ° C. 70 70 70 70 70 7070 80 80 70 Total pressure, kPa 250 250 250 250 250 250 400 300 300 400Ethylene, g 400.52 400.81 400.35 400.06 500.17 500.31 540.36 145.35145.21 485.78 Hydrogen, g 0 0 0 0 0 0 0 0 0 0 1-butene, g 0 0 0 0 18.8418.91 0 8 8 0 Third reactor Polymerization type Copo Copo Copo Copo CopoCopo Homo Copo Copo Copo Temperature, ° C. 70 70 70 70 70 70 80 80 80 70Total pressure, kPa 400 400 400 400 400 400 600 600 600 600 Ethylene, g500.74 500.11 500.30 500.63 600.02 601.19 720.60 580.53 580.46 647.54Hydrogen, g 0 0.001 0.001 0.001 0 0.001 0 0.59 1.37 0 1-butene, g 35.0530.01 30.03 30.04 60.01 60.04 0 27 27 20.60 W_(A)means percent by weightof Polymer in the first reactor W_(B)means percent by weight of Polymerin the second reactor W_(C)means percent by weight of Polymer in thethird reactor

TABLE 2 CE1 E1 E2 CE2 CE3 Powder MI₅, 0.474 0.372 0.240 0.242 0.275 g/10min MI₂₁, 13.83 10.80 7.38 7.23 6.40 g/10 min Density, 0.9565 0.95780.9555 0.9567 0.9441 g/cm³ IV, dl/g — — — — — Mw 276,413 244,279 291,295319,487 252,160 Mn 8,877 8,724 8,843 8,472 8,016 Mz 2,788,607 2,370,6783,401,041 4,135,007 1,638,224 PDI 31 28 33 38 31 Pellet MI₅, 0.436 0.4100.232 0.199 0.298 g/10 min MI₂₁, 14.46 11.68 7.876 6.696 7.485 g/10 minDensity, 0.9577 0.9574 0.9568 0.9566 0.9442 g/cm³ IV, dl/g 2.97 3.033.52 3.64 3.12 % Crystallinity, % 64.70 67.24 64.78 66.16 57.49 Charpy,23.5 29.9 35.3 30.5 47.9 23° C., kJ/m² Flexural 1,130 1,210. 1,123 1,123727 modulus, MPa E3 E4 CE4 E5 E6 Powder MI₅, 0.200 — 54.80 48.07 NA g/10min MI₂₁, 4.81 0.145 641 653 NA g/10 min Density, 0.9438 0.9534 0.96060.9590 0.9409 g/cm³ IV, dl/g — 9.00 1.07 1.06 23 Mw 306,468 868,81377,334 91,752 1,269,336 Mn 7,637 24,107 5,400 6,035 23,450 Mz 2,643,9535,112,060 667,276 1,027,956 5,262,195 PDI 40 36 14 15 54.13 Pellet MI₅,0.195 — 60.62 55.47 — g/10 min MI₂₁, 4.604 — 713.1 752.2 — g/10 minDensity, 0.9440 — 0.9608 0.9594 — g/cm³ IV, dl/g 3.37 9.00 1.0 1.1 23 %Crystallinity, % 54.05 68.23 69.52 65.64 58.20 Charpy, 50.9 84.4 1.5 1.885.41 23° C., kJ/m² Flexural 785 1,109 1,147 1,196 890 modulus, MPa

Film Related Examples

To prepare an inventive film from the above compositions, it was foundthat a sub-range of multimodal polyethylene compositions which might beobtained using the inventive reactor system are particularly preferred.In detail, the compositions suitable to form the inventive film are asfollows and have the following properties. The following comparativeexamples refer to the film related compositions.

The inventive example E7 was produced follow the inventive process tomake the multimodal polyethylene composition as shown in table 3. Thespecific multimodal polyethylene compositions enhance superiorproperties of film in particular the ability to make thin film. The thinfilm is represented the low thickness of the film such as 5 micron. Itcould be also refer to the ability to down-gauge the film thickness withequivalent properties to conventional film thickness.

The inventive example E8 is the multimodal polyethylene compositionproduced by inventive process and having polymer as shown in table 5 inthe range of claims with MI₂ of 0.114 g/10 min and density of 0.9570g/cm3. It shows good processing in film production and higher outputrate with maintaining properties in particular dart drop impact andpuncture resistance at 12 micron film thickness.

TABLE 3 Process condition of inventive example 7, E7 and E8 andcomparative example 6, CE7 Condition Unit CE7 E7 E8 1st Reactor Splitratio % 49-50 45-47 45-47 Temperature (° C.) 81-85 81-85 81-85 PressurekPa 700-750 650-700 580-620 Hydrogen flow rate NL/h 246 226 248 2ndReactor Split ratio % 6-8 10-12 10-12 Temperature (° C.) 70-75 70-7570-75 Pressure kPa 150-300 150-300 150-300 Hydrogen flow rate NL/h 0 0 0Co-monomer kg/h 0.031 0.010 0.0135 Comonomer/Ethylene Feed — 0.0180.0033 0.0046 H2 removal 99.0 98.9 99.4 Comonomer type — 1-Butene1-Butene 1-Butene 3rd Reactor Split ratio % 42-43 42-43 42-43Temperature (° C.) 70-75 70-75 70-75 Pressure kPa 150-300 150-300150-300 Hydrogen flow rate NL/h 12.85 13.02 17.28 Co-monomer kg/h 0.0520.0152 0.0099 Comonomer/Ethylene Feed — 0.0048 0.0013 0.0009 Comonomertype — 1-Butene 1-Butene 1-Butene

From the molding composition so prepared, a film was produced in thefollowing way. The films having different thickness and output wereprepared on the internal blown film machine comprising a single screwextruder connecting with tubular blow film apparatus. The temperaturesetting from extruder to the die is from 175 to 205° C. The screw speedand nip roll take up speed to prepare different film thickness in eachexperiment is defined in table 4. The film was produced at a blow-upratio of 4:1 and a neck height of 30 cm with bubble diameter of 23 cmand film lay flat of 39 cm.

TABLE 4 Experiment and conditions for film preparation Experiment 1Experiment 2 Experiment 3 Blown film parameter (Ex. 1) (Ex. 2) (Ex. 3)Film thickness 12 5 5 Screw speed (rpm) 85 85 60 Nip roll take up speed(rpm) 80 150 95 BUR 4:1 4:1 4:1 Neck height (cm) 30 30 30

The comparative example 4 (CE5) is the commercial resin EL-Lene™ H5604Fproduced by SCG Chemicals., Co. Ltd. with MI₂ of 0.03 g/10 min anddensity of 0.9567 g/cm³. It is the bimodal polyethylene produced inslurry cascade process.

The comparative example 5 (CE6) is the blend of CE4 with commercialresin LLDPE, Dow™ Butene 1211, with MI₂ of 1.0 g/10 min and density of0.9180 g/cm³. It is the practical way in film production to get betterfilm strength in particular dart drop impact and tear strength.

The comparative example 6 (CE7) is the multimodal polyethylenecomposition produced by the inventive process and having the compositionand molecular weight distribution out of the specific range ofcomposition for thin film.

The films were further evaluated for processability and mechanicalproperties in both machine direction, MD and transverse direction, TD asshown in table 5.

TABLE 5 Properties of polyethylene compositions and film thereofProperties CE5 CE6 CE7 E7 E8 Resin MI₂, g/10 min 0.03 0.065 0.08 0.080.114 MI₂ of LMW NA NA 624 715 722 Density, g/cm³ 0.957 0.952 0.9550.957 0.957 Density of HMW1, NA NA 0.921 0.924 0.921 g/cm³ Density ofHMW2, NA NA 0.946 0.947 0.947 g/cm³ Mn (g/mol) 7,788 8,298 9,579 9,0278856 Mw (g/mol) 240,764 276,362 284,257 232,875 228,400 Mz (g/mol)1,817,918 1,956,827 1,666,188 1,403,576 1,346,144 PDI 30.9 33.3 29.725.8 25.7 Melt strength at 0.28 0.25 0.22 0.26 NA break, N Draw downratio at 10.5 12.2 12.8 12.5 NA break Film Ex. 1 Ex. 2 Ex. 3 Ex1 Ex1 Ex.1 Ex. 2 Ex1 Output, kg/hr 16.0 NA 12.8 19.1 20.3 19.7 19.9 20.3 Filmthickness, 12 5 5 12 12 12 5 12 micron Screw speed, rpm 85 85 60 85 8585 85 85 Nip roll take up 80 150 95 80 80 80 150 80 speed, rpm Blow upratio, 4:1 4:1 4:1 4:1 4:1 4:1 4:1 4:1 BUR Bubble Stability Good BubbleGood Good Good Good Good Good Break Dart drop impact, g 105 — 113 140124 159 108 124 Tensile Strength at 722 — 889 428 537 895 1068 537 Break(MD), kg/cm² Tensile Stregnth at 501 — 574 320 537 745 499 537 Break(TD), kg/cm² Elongation at 266 — 52 161 226 417 192 226 Break (MD), %Elongation at 510 — 388 390 488 605 365 488 Break (TD), % Tear Strength4.14 — 8.4 7.8 5.5 6.6 2.3 5.5 (MD), g Tear Strength 50 — 14 49 26 60 2726 (TD), g Puncture Energy, 26 — 39 21 29 31 46 29 N-cm/u

The inventive example 7 shows superior properties of 12 micron filmprepared by the same conditions compared to comparative examples, CE5,CE6 and CE7. The inventive E8 shows maintain film property and higheroutput with good bubble stability. In particular dart drop impactstrength, tensile strength of film in both directions and punctureresistance. Also the film is produced with higher output.

Further experiment to make a thin film at 5 micron was performed inExperiment 2. The Inventive example E7 and E8 show better draw abilityat higher output which can be easily drawn into 5 micron film with goodbubble stability and good mechanical strength. The same experiment wasapplied to the comparative example CE5 however bubble break was suddenlyfound. It was possible to make the 5 micron film with CE5 only in thecase of lowering output by reducing screw speed and nip roll take upspeed as done in Experiment 3. This is also related to draw down atbreak measured by rheoten. The inventive example1 E7 and E8 has higherdraw down at break compared to comparative example CE5.

Moreover the properties of the 5 micron film made by inventive exampleE7 in Experiment 2 are also equivalent to 12 micron film made by CE5with Experiment 1 in particular dart drop impact strength, tensilestrength at break and puncture resistance. This also indicated theability to downgauge the film thickness without sacrifice of mechanicalproperties. It was also possible to obtain good mechanical propertieswithout use of LLDPE as compared to comparative example CE6.

These results support that the inventive multimodal polyethylenecomposition provide better balance of mechanical strength with highoutput for thin film preparation.

The features disclosed in the foregoing description and in the claimsmay, both separately and in any combination, be material for realizingthe invention in diverse forms thereof.

1. A reactor system for a multimodal polyethylene polymerizationprocess, comprising; (a) a first reactor; (b) a hydrogen removal unitarranged between the first reactor and a second reactor comprising atleast one vessel connected with a depressurization equipment, preferablyselected from vacuum pump, compressor, blower, ejector or a combinationthereof, the depressurization equipment allowing to adjust an operatingpressure to a pressure in a range of 100-200 kPa (abs); (c) the secondreactor; and (d) a third reactor.
 2. The reactor system according toclaim 1, wherein the depressurization equipment allows to adjust theoperating pressure in the hydrogen removal unit to a pressure in therange of 103-145 kPa (abs), preferably 104-130 kPa (abs), mostpreferably 105 to 115 kPa (abs).
 3. The reactor system according toclaim 1, wherein the hydrogen removal unit further contains a strippingcolumn for the separation of hydrogen and a liquid diluent.
 4. A processfor producing a multimodal polyethylene composition in the reactorsystem according to claim 1, comprising; (a) polymerizing ethylene in aninert hydrocarbon medium in the first reactor in the presence of acatalyst system, selected from Ziegler-Natta catalyst or metallocene,and hydrogen in an amount of 0.1-95% by mol with respect to the totalgas present in the vapor phase in the first reactor to obtain a lowmolecular weight polyethylene having a weight average molecular weight(Mw) of 20,000 to 90,000 g/mol or medium molecular weight polyethylenehaving a weight average molecular weight (Mw) of more than 90,000 to150,000 g/mol wherein the low molecular weight polyethylene,respectively the medium molecular weight polyethylene, has a density atleast 0.965 g/cm3, and the low molecular weight polyethylene has MI₂ inthe range from 10 to 1,000 g/10 min and the medium molecular weightpolyethylene has MI₂ in the range from 0.1 to 10 g/10 min; (b) removingin the hydrogen removal unit 98.0 to 99.8% by weight of the hydrogencomprised in a slurry mixture obtained from the first reactor at apressure in the range of 103-145 kPa (abs) and transferring the obtainedresidual mixture to the second reactor; (c) polymerizing ethylene andoptionally C₄ to C₁₂ α-olefin comonomer in the second reactor in thepresence of a catalyst system, selected from Ziegler-Natta catalyst ormetallocene, and in the presence of hydrogen in an amount obtained instep (b) to obtain a first high molecular weight polyethylene having aweight average molecular weight (Mw) of more than 150,000 to 1,000,000g/mol or a first ultra high molecular weight polyethylene in the form ofa homopolymer or a copolymer having a weight average molecular weight(Mw) of more than 1,000,000 to 5,000,000 g/mol and transferring aresultant mixture to the third reactor; and (d) polymerizing ethylene,and optionally C₄ to C₁₂ α-olefin comonomer in the third reactor in thepresence of a catalyst system, selected from Ziegler-Natta catalyst ormetallocene, and hydrogen, wherein the amount of hydrogen in the thirdreactor is in a range of 0.1-70% by mol, preferably 0.1-60% by mol withrespect to the total gas present in the vapor phase in the third reactoror optionally substantial absence of hydrogen to obtain a second highmolecular weight polyethylene having a weight average molecular weight(Mw) of more than 150,000 to 1,000,000 g/mol or a second ultra highmolecular weight polyethylene in the form of a homopolymer or copolymerhaving a weight average molecular weight (Mw) of more than 1,000,000 to5,000,000 g/mol.
 5. The process according to claim 4, wherein theremoving is removing of 98.0-99.8% by weight of the hydrogen, morepreferable 98.0-99.5% by weight, and most preferred 98.0 to 99.1% byweight.
 6. The process according to claim 4, wherein the operationpressure in the hydrogen removal unit is in the range of 103-145kPa(abs), more preferably 104-130 kPa (abs), and most preferred 105 to115 kPa (abs).
 7. A multimodal polyethylene composition obtainable by aprocess according to claim 4, comprising; (A) 30 to 65 parts by weight,preferably 40 to 65 parts by weight, preferably 43 to 52 parts byweight, most preferred 44 to 50 parts by weight, of the low molecularweight polyethylene, the low molecular weight polyethylene having aweight average molecular weight (Mw) of 20,000 to 90,000 g/mol andhaving a MI₂ from 500 to 1,000 g/10 min according to ASTM D 1238; (B) 8to 30 party by weight, 8 to 20 parts by weight, preferably 10 to 18parts by weight, most preferred 10 to 15 parts by weight, of the firsthigh molecular weight polyethylene having a weight average molecularweight (Mw) of more than 150,000 to 1,000,000 g/mol or the first ultrahigh molecular weight polyethylene having a weight average molecularweight (Mw) of more than 1,000,000 to 5,000,000 g/mol; and (C) 30 to 50parts by weight, preferably 37 to 47 parts by weight, most preferred 39to 45 parts by weight, of the second high molecular weight polyethylenehaving a weight average molecular weight (Mw) of more than 150,000 to1,000,000 g/mol or the second ultra high molecular weight polyethylenehaving a weight average molecular weight (Mw) of more than 1,000,000 to5,000,000 g/mole, wherein the density of the first high molecular weightpolyethylene or the first ultra high molecular weight polyethylene andthe second high molecular weight polyethylene or the second ultra highmolecular weight polyethylene are in the range from 0.920 to 0.950g/cm³, and wherein the molecular weight distribution of the multimodalpolyethylene composition is from 13 to 60, preferably 20 to 28,preferably from 24 to 28, measured by gel permeation chromatography. 8.The multimodal polyethylene composition according to claim 7, whereinthe MI₂ is from 600 to 800 g/10 min.
 9. The multimodal polyethylenecomposition according to claim 7, wherein the molecular weightdistribution is from 23 to 28, preferably from 24 to 26, and morepreferably from 25 to 26 measured by gel permeation chromatography. 10.The multimodal polyethylene composition according to claim 7, whereinthe multimodal polyethylene composition has a weight average molecularweight from 80,000 to 1,300,000 g/mol, preferably 150,000 to 400,000g/mol, preferably from 200,000 to 350,000 g/mol, measured by GelPermeation Chromatography.
 11. The multimodal polyethylene compositionaccording to claim 7, wherein the multimodal polyethylene compositionhas a number average molecular weight from 5,000 to 30,000 g/mol, 5,000to 15,000 g/mol, preferably 7,000 to 12,000 g/mol, measured by GelPermeation Chromatography.
 12. The multimodal polyethylene compositionaccording to claim 7, wherein the multimodal polyethylene compositionhas a Z average molecular weight from 900,000 to 6,000,000 g/mol,1,000,000 to 3,000,000 g/mol, preferably from 1,000,000 to 2,500,000g/mol, measured by Gel Permeation Chromatography.
 13. The polyethylenecomposition according to claim 7 wherein the multimodal polyethylenecomposition has a density from 0.950 to 0.962 g/cm³, preferably from0.953 to 0.959 g/cm³, according to ASTM D 1505 and/or a melt flow indexMI₅ from 0.01 to 50 g/10 min, and/or MI₂ from 0.03 to 0.15 g/10 minpreferably from 0.03 to 0.10 g/10 min.
 14. The polyethylene compositionaccording to claim 13, wherein the MI₅ is from 0.01 to 1 g/10 min. 15.Film comprising the multimodal polyethylene composition according toclaim 7, wherein the film has a thickness from 4 to 40 μm, preferablyfrom 4 to 30 μm, and most preferably 4 to 20 μm.